He says the habitat of a nationally significant koala population will be bisected by the chosen route.

Dr Phillips says fencing and fauna crossings being proposed by the State Government are inadequate.

"This is one of those instances where the population is so significant that that sort of stuff is window dressing," he said.

"It moves away from the central issue of the need to look at the context of this population at a much bigger scale than just that road footprint.

"When you do that, you realise the long-term prognosis for this population is now very, very poor should this road go ahead."

But Bob Higgins, who manages the Pacific Highway upgrade for the Roads and Maritime Service, says a number of plans are in place to protect the koalas.

"The route that was selected is on mostly cleared land," he said.

"It's between the Blackwall Ranges and the Wardell heathland.

"What we're doing is fencing the full route, so koalas can't get onto the new highway, extensive fauna underpasses and a fauna overpass, and planting upwards of about 130 hectares of koala food trees."

The Federal Environment Minister still has to sign off on the project, which has been given state planning approval.